1. Field of the Invention
This invention relates to a process of making more valuable products from a low value petroleum refinery stream such as a vacuum residuum, and more particularly relates to a novel process for producing premium delayed coke from a petroleum residuum feedstock.
2. Description of the Prior Art
There are several processes available in the petroleum refining art for upgrading heavy, low value petroleum residual oils. Typical of such low value residual oils is the bottoms fraction from a vacuum distillation tower. Such vacuum distillation towers generally are used to further fractionate virgin atmospheric reduced crude oils. The bottoms fraction from such vacuum distillation columns, referred to herein as resid, generally includes all the material boiling above a selected temperature, usually at least 480.degree. C., and often as high as 565.degree. C. In the past, vacuum residuum streams have presented serious disposal problems, as it has been difficult to convert such streams to more valuable products in an economic manner. One method of disposing of resid has been to use the stream as feedstock to a fluid bed or delayed coking unit. The resulting coke generally has only fuel value. Fluid bed and delayed coking processes for converting vacuum residuum into coke are well known in the petroleum refining industry, and many commercial units utilizing these processes exist.
Delayed coking of vacuum residuum generally produces a coke with a linear coefficient of thermal expansion (CTE) greater than 20.times.10.sup.-7 .degree. C. The CTE of the coke is a measure of its suitability for use in the manufacture of electrodes for electric arc steel furnaces. The lower CTE cokes produce more thermally stable electrodes. Coke which is suitable for manufacture of electrodes for steel furnaces is generally designated as premium coke. Premium coke differs both in appearance and in physical properties from regular coke. The distinction between regular coke and premium coke was first described in U.S. Pat. No. 2,775,549 to Shea, although the needle coke described in that patent would not be acceptable in the present premium coke market. The manufacture and properties of premium coke are further described in U.S. Pat. No. 2,922,755 to Hackley.
The CTE value required for a coke to be designated premium coke is not precisely defined, and there are many specifications other than CTE which must be met in order for a coke to be designated premium coke. Nevertheless, the most important specification, and the one most difficult to achieve in manufacture, is a suitably low CTE. For example, the manufacture of 61 cm diameter electrodes requires CTE values of less than 5.times.10.sup.-7 .degree. C., and the manufacture of 41 cm diameter electrodes generally requires a coke having a CTE of less than 8.times.10.sup.-7 .degree. C. Delayed coking of vacuum residuum from most crudes produces a coke with a CTE of greater than 20.times.10.sup.-7 .degree. C., and such cokes, designated regular grade cokes, are not capable of producing a satisfactory large diameter graphite electrode for use in electric arc steel furnaces.
As used herein, the term "premium coke" is used to define a coke produced by delayed coking, which, when graphitized according to known procedures, has a linear coefficient of thermal expansion of less than 80.times.10.sup.-7 .degree. C. The process conditions for operating a premium delayed coking unit are well known in the art, and do not constitute a part of this invention. The conventional conditions for making delayed premium coke comprise introducing a premium coke feedstock to a coker furnace where it is heated to coking temperature, generally from 425.degree.-540.degree. C., followed by introduction of the heated feedstock to a delayed coking drum maintained at typical premium coking conditions of from 440.degree.-470.degree. C. and 0.5 to 7 kg/cm.sup.2. The procedure for making a graphitized electrode from premium coke is also well known in the art and does not constitute a part of this invention.
Another process which is available in the art for upgrading heavy, low value petroleum residual oils is hydrogen donor diluent cracking (HDDC). In this process, a hydrogen-deficient oil such as vacuum residuum is upgraded by admixing it with a relatively inexpensive hydrogen donor diluent material and thermally cracking the resulting mixture. The donor diluent is preferably an aromatic-napthenic material having the ability to take up hydrogen in a hydrogenation zone and to readily release it to hydrogen-deficient hydrocarbons in a thermal cracking zone. The selected donor material is partially hydrogenated by conventional methods, using, for example, a sulfur-insensitive catalyst such a molybdenum sulfide, nickel-molybdenum or nickel-tungsten sulfide. Details of the HDDC process are described in U.S. Pat. Nos. 2,953,513 and 3,238,118. A process for producing premium coke from vacuum residuum in which the vacuum residuum is subjected to a hydrogen donor diluent cracking process and the pitch from the cracking step utilized as feedstock to a delayed premium coker is described in Application Ser. No. 908,333 filed May 22, l978.
Catalytic desulfurization of high sulfur petroleum streams is well known, and is widely practiced in the petroleum refining art. Examples of processes utilizing catalytic desulfurization of petroleum streams are described in U.S. Pat. Nos. 2,703,780 and 2,772,221. The latter patent describes a process wherein resid is subjected to hydrogen donor diluent cracking, and a portion of the products from the cracking step is subjected to hydrogenation and subsequently used as recycle donor diluent. The conditions required to obtain a desired degree of hydrogenation are easily determined by those skilled in the art.
Prior to this invention, there has been no satisfactory process available which could produce a premium coke from a high sulfur vacuum residuum.